The present invention generally relates to a woven fabric which is designed for use in a papermaking, cellulose or board manufacturing machine and which along each one of two end edges has a plurality of loops to be included in a loop seam to form an endless woven fabric. The invention also relates to a method of manufacturing such a fabric.
More specifically, the invention provides an improvement of such a loop seam by conforming it to the rest of the woven fabric, so as to avoid markings in a paper web at the loop seam.
The woven fabric according to the invention is particularly useful, but by no means exclusively so, as a base fabric in a press felt for the press section of a papermaking machine. Press felts are manufactured either with or without a seam. Present-day Joining techniques rely almost exclusively on a so-called loop seam, where two fabric edges (not necessarily of the same fabric) to be joined together each have one row of seam loops. These loops are inclined or orthogonal to the principal plane of the fabric. When joining together the two fabric edges, the loops on one edge are inserted between the loops of the other edge, whereupon at least one separate seam thread is inserted in the interlaced seam loops to lock them to one another.
Such a loop seam is however not entirely satisfactory for several reasons, which will be given hereinafter with reference to FIGS. 1-7 illustrating a woven fabric of the prior art, a method of manufacturing a fabric, as well as the shortcomings of this prior art.
FIG. 1 is a vertical cross-section taken parallel to warp threads in an embodiment of a known double-layered woven fabric, and FIG. 2 is a section taken along the line II--II in FIG. 1. The woven fabric in FIG. 1 has warp threads V1, V2, V3, V4 extended in the plane of the drawing sheet, and weft threads 1-8 extended orthogonally to the plane of the drawing sheet and distributed in two layers L1, L2. The warp threads V1-V4 are crimped around the relatively straight weft threads 1-8, each warp thread, such as the warp thread V1, cyclically following the pattern "over.fwdarw.between.fwdarw.under.increment.between" with respect to the two layers L1, L2 of weft threads (see also FIG. 8 to the left of plane A).
FIG. 3 schematically shows a method of weaving the double-layered fabric in FIGS. 1 and 2 with a so-called round weaving technique, where seam loops 11, 11' of the above-mentioned type are woven simultaneously with the fabric. The weft in FIG. 3 is woven in the order 1.fwdarw.2.fwdarw.2'.fwdarw.1'. The weft threads 1 and 2 in the so-called top cloth form the seam loop 11 around a seam thread 10 parallel to the warp threads. The weft threads 1' and 2' in the so-called bottom cloth form the seam loop 11' around the same seam thread 10. At the transition to the left in FIG. 3 between the top cloth and the bottom cloth, the weft forms an irregularity at the loom edge.
FIG. 4 schematically shows on a larger scale the seam loop 11 with the seam thread 10 removed, and the four warp threads V1-V4 located closest to the loop 11.
FIG. 5 schematically shows a finished loop seam between the end edges of a double-layered woven fabric according to FIGS. 1-4. As described above, the seam loops 11, 11' are formed by weaving the weft threads 1-8 around the seam thread 10. The seam thread 10 used during the weaving procedure and hereinafter referred to as "weaving seam thread", may typically have a diameter of 1.2-1.7 mm. The weaving seam thread is removed from the seam loops before the woven fabric is mounted in the papermaking machine. For the final joining of the seam in the papermaking machine, use is however normally made of a seam thread 10 of slightly smaller diameter, e.g. 0.7 mm, to enable it to be passed easily through the loops. The area around the final loop seam (FIG. 5) will therefore have a larger void as compared with the seam formed directly in the loom (FIG. 3). This increased void is illustrated in FIG. 5, where the seam zone consists of regions O1, O2, O1, where O2 is the region occupied by the seam thread 10 of the final loop seam, and the two regions O1, O1 on each side of the O2 region representing the part of the loops 11, 11', respectively, which gives rise to said increased void of the seam zone. In FIG. 6, which illustrates the seam of FIG. 5 in a still more simplified view, the woven fabric composed of the warp threads and the weft threads are generally designated 20.
The following problems P1-P4 are encountered in the prior art:
P1 The seam zone does not have the same water permeability as the rest of the woven fabric because the seam loops 11, 11' have in the O1 areas a larger void than the rest of the fabric. If the fabric is used as a base fabric in a press felt, this may lead to an undesired marking on the paper web, being plastic during pressing, as a result of different dewatering in the seam zone. PA1 P2 If the woven fabric is used as a base fabric in a press felt which on its paper side (i.e. the side facing the paper web during operation) has a relatively compressible top layer of a batt needled to the base fabric, the batt will become anchored less efficiently in the seam zone (O1-O2-O1) with a consequent risk that it may easily be worn away and undesired markings may occur in the paper web. PA1 P3 The increased void of the seam zone in the O1 areas results in higher air permeability, entailing the following problems. If the poorly anchored batt portion as stated under P2 above is designed according to FIG. 7 as a flap 22 covering the seam zone (this technique is described in SE 8206222-5), this batt flap 22 will, when the seam zone of the press felt passes a dewatering suction box (not shown) disposed on the batt side of the felt, whip into the suction box, producing a pistol-shot-like sound, and be subjected to wear, as schematically illustrated by the dash-dot lines in FIG. 7. PA1 P4 When a press felt with a base fabric according to FIG. 5 passes a press nip, there will occur at the seam zone a variation in compressibility, producing a marking in the paper. From FIG. 5 appears that the thickness of the base fabric in the seam zone (O1-O2-O1) is 2.times..increment.T less than the thickness T of the rest of the fabric, where .increment.T corresponds to the warp thread diameter which is e.g. 0.4 mm. A batt layer on the paper side of the base fabric will therefore exhibit a reduced thickness in the press nip. The increased voids within the seam loops at the O1 regions also contributes to the compressibility variation.
FIG. 5 indicates by dashed lines at 26 a known technique for reducing the void in the regions O1 of the seam zone. After the woven fabric has been joined together by means of the seam thread 10 in the papermaking machine, one or more filling yarn threads 26 are passed through the seam loops in the regions O1. The use of such threads 26 reduces to some extent the problems P1 and P3 stated above (relating to deviating water and air permeability, respectively). The use of filling yarn threads does however not solve problem P2 (poor batt anchorage), since the threads 26 are inserted after the needling of the batt, or problem P4 (reduced thickness), since the filling yarn threads 26 are completely within the seam loops 11, 11' and therefore cannot eliminate the thickness reduction 2.times..increment.T. Moreover, the filling yarn threads pose per se an additional problem (P5), because the technique is time-consuming, which is especially serious in a papermaking machine where downtime is highly detrimental for cost-efficiency reasons.
It appears from the above that the O1 regions of the seam zone are undesirable. Reducing the O1 regions by making the seam thread 10 for the final joining of the seam thicker is however no viable solution to the problem. First, the seam thread will become difficult or impossible to insert when Joining the fabric ends together in the papermaking machine. Second, a seam thread which is too thick may entail an unacceptably high density in the O2 region. This is because the seam thread in itself is thicker than the warp threads and because the weft density normally is twice as high in the O2 region, since the loops in this region are interlaced side by side against each other.